For many years significant efforts have been made to develop systems for reliably establishing the identity of a person. Such investigations have produced systems which automatically evaluate fingerprints, retina patterns and voice samples. Another area which has undergone significant investigation is dynamic signature analysis.
Dynamic signature analysis relates to the evaluation of the real time movements of a subject's hand during the signature process. This should be contrasted with a static signature analysis which merely compares the appearance of the end result of the signature with a known prior handwritten sample. While the latter approach has some use, it is not particularly secure and can be defeated by the steady hand of a forger. As can be appreciated, a forger can merely study the end result of the signature and produce a slow but faithful copy of that signature.
Dynamic signature verification provides an entirely different approach. Briefly, a means is provided for measuring the forces applied to a stylus during the signature. These forces are typically measured in the two orthogonal directions (x and y) and in the vertical or downward (z) direction. These pressure measurements can be taken through transducers mounted in the pen or utilizing a force sensitive pad.
During the course of a signature, measurements are taken which are proportional to the pressure exerted by the subject. These signals will vary as the pen is moved through the curves and breaks of the signature. If this information is stored, it can be compared with data measured during a subsequent signature and a determination can be made if the signals are similar and fall within certain defined limits. Experiments have shown that it is very difficult for one person to emulate the pressure variations of another. As can be appreciated, a typical forger, working carefully to copy a signature, will not utilize the same speed or stroke sequence as the original signer. Accordingly, it is believed that dynamic signature analysis provides an excellent approach for identifying authorized personnel.
One of the first systems for implementing dynamic signature verification is described in U.S. Pat. Nos. 3,528,295 issued Sept. 15, 1970 and 3,579,186 issued May 18, 1972, both to Johnson et al. These patents disclose both a stylus and circuitry for performing dynamic signature analysis. More specifically, U.S. Pat. No. 3,528,295 discloses a stylus having pressure responsive transducers mounted therein. In a first, relatively simple embodiment of the stylus, a single transducer is provided for measuring downward (z-axis) pressure. In operation, signals generated during a first, reference signature would be stored and compared with signals generated during a subsequent signature to evaluate the identity of the writer.
In a more sophisticated embodiment, four transducers are provided, oriented in an orthogonal relationship to provide information in three axes. During a signature, output signals would be generated corresponding to the pressures exerted in the left to right directions (x-axis), far to near (y-axis) and downward (z-axis). Where signals are measured in a plane (x,y), the orientation of the pen with respect to the subject's hand becomes significant. As can be appreciated, if the pen were rotated between signatures, the forces measured on the specific transducers would be completely different. In this case, output signals generated during two identical signatures would not match and an authorized user would be rejected.
One approach for avoiding this particular problem is to measure the forces on a tablet rather than through the stylus. For example, in U.S. Pat. No. 3,988,934, issued Nov. 2, 1976 to Kamphoefner et al, a platen is disclosed which can measure the pressure applied by a stylus in the x,y directions. These measurements are, of course, independent of the rotational position of the stylus with respect to the writer's grip. The latter approach, however, requires that the subject place the stylus at the same location on the platen and write in the same direction each time. In addition, it has been found that subjects often rest their fingers or hands on the platen, distorting the output signals.
As can be appreciated, utilizing a platen to measure stylus pressures also has many shortcomings. Therefore, further efforts have been made to develop a stylus system which would generate unambiguous (x,y) information. This result was typically achieved by providing the stylus with a configuration that ensures it will always be gripped in the same manner. As set forth in the Johnson U.S. Pat. No. 3,528,295, the stylus housing is designed with special thumb and/or finger position guides. In addition, the stylus housing may be provided with an elliptical or oval shape in cross-section, to aid in properly orientating the stylus in the preferred position.
The above structural type of solution to the problem is adopted in various forms in U.S. Pat. No. 3,145,367 issued Aug. 18, 1964 to Crane, U.S. Pat. No. 3,986,403 issued Oct. 19, 1976 to Hurd et al., U.S. Pat. No. 4,087,226 issued Mar. 7, 1978 to Eer Nisse et al., Reissue Pat. No. 29,795 issued Sept. 9, 1978 to Crane et al. Both the Crane and Hurd patents disclose pens that have a unique configuration which purports to restrict variations in the manner that a subject grips the pen. An illustration of this type of stylus is shown in FIG. 1, discussed in greater detail below. In the patent to Eer Nisse, an alternate solution to the problem is suggested wherein printing is provided on the pen that states "this side up". Unfortunately, the latter solutions are not fully desirable. Clearly, the mere legend "this side up" would not prevent an unknown user or a careless user from improperly holding the pen. The configurations suggested in the Crane and Hurd patents have turned out to be significantly difficult to handle, particularly where the signor is unaccustomed to such odd shapes. Furthermore, a particular stylus shape tends only to influence the user into selecting a similar grip. Unfortunately, even slight changes in pen position will effect the output signals generated and adversely effect the accuracy of the system.
Accordingly, it is an object of the subject invention to provide an entirely different solution to the problem addressed in the above-cited patents.
It is a further object of the subject invention to provide a new and improved method of dynamic signature verification which permits the subject to grip a stylus in any position with respect to his hand.
It is another object of the subject invention to provide a new and improved method of signature verification wherein a simplified straight stylus configuration may be utilized.
It is still another object of the subject invention to provide a new and improved signature verification method wherein the average direction of the signature is computed and utilized to compensate for the varying rotational positions of the stylus with respect to the user's hand.
It is still a further object of the subject invention to provide a new and improved method of dynamic signature verification wherein the signals generated by the pen are normalized such that they are insensitive to the rotational position of the stylus with respect to the writer's grip.